There are a lot of patents and literatures currently available about the battery equalizing and matching, equalization of protection board, equalization of charger, and equalization of power supply system. A Chinese patent application with the Application No. 201110186716.1, introduced a consistency screening method for the lithium secondary batteries. Charging the battery to a low state of charge via the approach of multi-stage constant current and constant voltage, and laying for a suitable period of time, selecting the battery cell whose voltage is within a certain range to be used for the battery group matching. However, no matter how the battery cells are selected, two batteries always have some difference between them. So, if the requirement for consistency is set too high, the pass rate of the battery available for matching will be low; while if the requirement for consistency is set too low, the reliability of the matched battery group will be low. So, it needs to weigh the choices between the two factors.
Currently, the control parameters for the grade dividing and group matching of battery cell comprise capacity, voltage, internal resistance, and etc. In regard to the control for battery self-consumption, it usually set the state of charge of battery cells as full charge SOC 100% or half charge SOC 50%, and then lay them at room temperature or at 45˜50° C. for a pre-determined period of time, and the battery cells with the voltage greater than or equal to the pre-determined standard value are the qualified. Alternatively, dividing the grade based on the voltage, i.e., measure the battery cell voltage and then divide the grade accordingly. The grade dividing based on the voltage has a certain degree of control to the level of battery self-consumption. However, due to the fact that after charging or discharging, the voltages of various battery cells are different themselves, which is because of the difference of various battery cells and the difference of the test cabinet points. Since the voltages of the battery cells are different from themselves, the grade dividing based on the voltage cannot accurately divide the grades according to the battery self-consumption.
Among the currently available battery self-consumption testing and sorting system, some of them accomplish the grade dividing via the voltage difference; i.e., after measuring the battery voltage, lay it aside for a pre-set period of time, and then measure its voltage again, and based on the difference of the voltage before and after to conduct the grade dividing. Such grade dividing based on the voltage difference is a progress from the grade dividing that is based on the voltage. However, due to the facts that the battery cells from different grades of voltage difference usually have different self-consumption or consumable current (or sometimes called self-discharge current); and sometimes such difference is quite large, the magnitude of the consumable current and the range of the consumable current of individual battery cell are actually unknown. So, it cannot design and clearly control battery group's laying service life. The standard method for measure the self-consumption is, according to the national or industry standard, measuring the charge retention capability of a battery. After the standard charging and discharging for 1 week with the battery or battery measure out the standard capacity C5, next perform standard charging, and then under the condition of the environmental temperature at 23° C.±2° C., lay it aside as open circuit for 28 days, next discharge it with 0.2 C5 mA to the cut-off voltage, and measure out the capacity after the laying C5′, so the charge retention capability=C5′/C5×100%. The lost part is the self-consumption rate=(C5-C5′)/C5×100%. In the industry, the processes of battery shelving, storing and assembling are usually operated at the state of full charge or half charge. Within the range of SOC 50˜100% the voltage changes of certain types of batteries are quite insignificant, such as the fully charged 10 Ah lithium iron phosphate-graphite battery, following charging, its open circuit stable voltage is 3.338 V, then discharging for 1 h with 1 A, 1 Ah has been discharged, the SOC is 90% and the open circuit stable voltage changes to 3.334 V; then discharge it again with 1 A, and another 1 Ah has been discharged, SOC becomes 80% and its open circuit stable voltage changes to 3.332 V. Although the currently available voltage measurement technology is able to measure with the accuracy up to 0.1 mV, or even higher accuracy; as for the large scale production in the factory, the environmental temperature, air direction from the conditioner, pressure of the measurement probe, the contacting position, and etc, may all influence the measurement accuracy for the voltage. Or the values shown in the measurement keep randomly changing, which makes it hardly can be determined manually; even for a machine, it can only made the determination randomly. So, the actual accuracy may be just at 0.1˜1 mV.
In the battery group, there are always some differences of the consumable current between individual battery cells. During the processes of storing, shelving, placing, and using, due to the difference of the consumable current, the consistency among the individual battery cells, in particular the state of charge (SOC), keeps changing, and such difference gradually becomes bigger. Thus over a period of time, the battery group is going to fail. A battery group needs to be equipped with the equalization protection board. During the process of charging, it makes the states of charge of various individual battery cells become consistent. In the case when the equalization current of the equalization protection board does not match the range of the consumable current of the battery cells, and the range of the consumable current of the battery cell is too large, during charging process, it cannot make the states of charge of various individual battery cells become consistent within a reasonable period of time. In addition, if the charger does not match the equalization capability of the equalization protection board, and the charging cut-off current is much larger than the equalization current of equalization protection board, then in such a case, when the equalization is just started or even has not started yet, the charging is already terminated, then during the process of charging, it cannot make the states of charge of various individual battery cells become consistent, either.
There are mainly two equalization methods currently available. One method is the passive equalization, which only functions during the charging process. When a certain battery cell reaches the equalization initiation voltage, a part of the current will be leaked via a bypass resistor, thus to reduce the charging speed of the battery cell of a high voltage, and allow it to wait for the voltage increase of the battery cell of a low voltage. During the equalization process, it will generate heat; and its equalization efficiency is low. Yet its main advantage is the low price tag. This is the widely utilized equalization method. The other one is the active equalization method. It extracts energy from the battery cell with a high voltage via an inductance coil, and then transports it to the battery cell with a low voltage. Thus it has a very small energy loss; and generates almost no heat; as well as has a high equalization efficiency. This is a very good equality method. Nevertheless, its equalization cost is too high, which could be 3˜5 fold of that of the passive equalization method.
The currently available equalizing and matching methods for the lithium secondary battery have certain drawbacks. They cannot ensure that the range of the consumable current of the battery cell within a controllable range; and cannot make a relational design of the range of the consumable current of the battery cells, the equalization current of the equalization protection board and the charging cut-off current of the charger; and also cannot effectively control the laying service life of a battery group.